1. Field of the Invention
The invention relates to an arrangement for the lateral positioning of a recording medium in a printer or photocopier, preferably an arrangement for the positioning of endless paper in the fuser, or fixer, station of an electrophotographic printer.
2. Description of the Related Art
It is customary to use endless paper with transportation perforations in high speed non-mechanical printing apparatuses which operate by the electrophotographic or magnetic principle. In this case, the endless paper is withdrawn from a stack, transported through a transfer station with the aid of tractors engaging in the edge perforations and thereby coated with a layer of toner depending on the characters to be printed. The layer of toner situated on the recording medium is then thermally fused in a fuser station. A fuser station of this type is known, for example, from U.S. Pat. No. 4,147,922. For this purpose, the fuser station comprises in a customary manner an electric, heated fuser roller and a feed roller which can be pivoted onto and away from the fuser roller. In this case, the recording medium must be guided precisely through the fuser station without movement relative to the fuser roller. As s result of the design, the fuser station is arranged at a relatively large spacing from the transfer station and the paper tractors situated there which guide and transport the web of paper. There is thus the risk of the web of paper migrating horizontally in the fuser station during the printing operation. This can lead to tensioning of the web of paper and, with high transport speeds, to a tear in the paper.
Additionally, non-mechanical printing apparatuses must be designed in such a way that they can process webs of recording media of different widths. With these, it is necessary to design the fuser station with the rollers arranged therein in accordance with the greatest width of recording medium. Regions of the fuser roller or of the feed roller in the fuser station which do not come into contact with the recording medium heat up locally more intensely than the actual contact regions with the recording medium because the recording medium withdraws heat from the fuser roller in the fusing region during the fusing operation. This leads to a heat-dependent different diameter of the feed roller which generally consists of elastic material. As a result, the recording medium is deflected horizontally.
It has therefore proved necessary to monitor the lateral position of the recording medium precisely when it enters the fuser station in order to be able to intervene, if necessary, in the control of the fuser.
However, the problem of precise controllable guidance of recording media is present not only in electrophotographic printers and photocopiers. Even with other printing apparatuses which operate, for example, with inking units, it is necessary to position the recording medium precisely, at least in the printing region.
For positioning the recording media in printing apparatuses, it is customary to use electrically adjustable mechanical guide elements in the paper duct of the printing apparatus, the scanning of the ACTUAL position of the recording media likewise taking place via mechanical contact elements.
For scanning the position of the recording media, it is known from the IBM Technical Disclosure Bulletin, Vol. 23, No. 7a, December 1980 and from European Patent 0 031 137 to use optoelectronic scanning apparatuses.
A further problem consists in the scanning of the ACTUAL position of the recording media with the aid of such optoelectronic scanning apparatuses. In modern electrophotographic and magnetic printers, different recording medium materials can be used regardless of the scope of application. These recording media can consist, for example, of strongly reflecting white paper or a film with a metallized surface or even of transparent plastic film. In all these cases, it is difficult due to the different transparency of the materials used to scan the edge of the recording medium or its transportation perforations using an optoelectronic scanner.